Electron tube



June 5, 1962 c. D. O'NEILL ELECTRON TUBE Filed July 14, 1959 INVENTIORGEORGE 0. ONE/LL ATTORNE 3,038,098 Patented June 5, 1962 3,038,098ELECTRON TUBE George D. ONeill, Roslyn, N.Y., assignor to SylvaniaElectric Products Inc., a corporation of Delaware Filed July 14, 1959,Ser. No. 827,110 3 Claims. (Cl. 313-151) My invention is directed towardelectron tubes.

Conventionally, the transconductance of an electron tube having closelyspaced electrodes (for example, cathode-grid and anode-grid spacings ofa few thousandths of an inch) remains constant only when these spacingsand the cathode heater power of the tube do not vary. However, when theheater power varies, the dissipation varies and, therefore, theelectrode spacings may vary. In addition, the variations in heater powerproduce variations in cathode temperature, and this would result invariations in t-ransconductance even if the interelectrode spacing were,by some means, held constant. In general, the change in transconductanceproduced by changes in spacing is in the same direction as that producedsolely by a change in cathode temperature. Therefore the change intransconductance is the sum of the changes due to the separate effects.

In many applications, it is highly desirable to employ tubes havingclosely spaced electrodes wherein the cathode heater power can vary overa relatively wide range, while the tube transconductance remainsessentially constant. Many attempts have been made to produce such tubesbut, in so far as I am aware, such attempts have been unsuccessful. Incontradistinction, I have succeeded in producing such a tube. V

In accordance with the principles of my invention, my tube comprisesessentially, in an evacuated envelope, an anode electrode, a controlgrid spaced apart from said anode, and a cathode electrode spaced apartfrom said grid. One or both electrodes are mounted on members formedfrom materials which either contract or do not expand appreciably as theambient temperature increases; i.e, materials that possess either anegative coefficient of thermal expansion or an essentially zeroexpansion coefficient.

As explained in more detail hereinafter, when electrode support membershaving an essentially zero thermal coefficient are employed in myinvention, the transconductance variations with changes in heater powercan be substantially reduced. Further, when one or more members havingnegative coeflicients of thermal expansion are utilized, thesetransconductance variations can be substantially eliminated.

Illustrative embodiments of my invention will now be described withreference to the accompanying drawings wherein:

FIG. 1 shows one embodiment of my invention utilizing electrode supportmembers having an essentially zero coeficient of thermal expansion; and

FIG. 2 shows another embodiment of my invention in which one of theelectrode support members has a negative coefiicient of thermalexpansion.

FIG. 1 shows a triode having small electrode spacings and useful as anoscillator or amplifier at very high fre quencies. This triode has anevacuated glass envelope 10. An indirectly heated cathode is positionedwithin the envelope and, for example, is constituted by a nickel cap 22which is coated with a conventional electron emissive material and whichis fastened to a first tubular support member 20 (normally of copper). Acathode heater in the form of a tungsten wire coil 24 coated withaluminum oxide is positioned within member 20. A grid 14, welded to disc(usually of copper) which separates the upper and lower parts ofenvelope 10, is positioned parallel to and closely spaced from thecathode. An

anode 18 is positioned above grid 14 and is welded to a second tubularsupport member 16.

In operation, input and output resonant cavities are placed around thetwo halves of the tube; the input cavity is coupled between the griddisc 15 and tubular member 20, while the output cavity is coupledbetween the grid disc and tubular member 16.

It is well known to the art that when a tube such as that shown in FIG.1 is operated, a change in heater power produces a change intransconductance. This change in transconductance can be intolerablylarge when the tube is operated from a power supply that may permitconsiderable change in heater power. For example, it is not uncommon fora 10 percent change in heater voltage to produce a 20 percent change intransconductance in tubes of this class when constructed as heretofore.The transconductance increases or decreases as the heater powerincreases or decreases.

This change in transconductance with heater power will be explainedbriefly below. (A more detailed and somewhat mathematical treatment ofthe factors causing the shift in transconductance with cathodetemperature can be found in my paper, The Influence of the InternalCorrection Voltage on the Proper Ratings of Receiving-Type Tubes,published in The Sylvania Technologist, vol. 10, No. 3, July 1957, pp.71-77, and in the IRE Transaction on Electron Devices, vol. ED5, No. 2,April 1958, pp. 69-75.)

When the heater power is increased, tubular member 20 is increased inlength because of increased temperature, and as a result the spacing sbetween cathode 22 and grid 14 is decreased. From Childs law, it isknown that the space current i is thereby increased considerably; withno other change, the current is proportional to 1/ s and thetransconductance g is also proportional to 1/S12.

If the heater power is abnormally high, the temperature of tubularmember 16 also rises, and this produces a decrease in the distance sbetween grid 14 and anode 18. This also causes an increase in i and gbecause the amplification factor of the tube is reduced. (Moreparticularly, t varies directly with spacing s Further, the spacechargecurrent i depends upon the amplification factor a in approximateaccordance with the equation in which G is the perveance, e, is the gridbias, and e is the anode voltage. Hence, as shown in this equation, whena is decreased, i is increased. Furthermore, if i is increased, then gis increased, since g is the partial derivative of the current i withrespect to the grid voltage e To circumvent the increase in g withdecrease in the spacing s due to an increase in heater power, I use amaterial for that part of tubular member 20 within the envelope whichhas substantially Zero coefficient expansion. As a result of thisconstruction, the change in g with change in heater power is reduced toa relatively small value, but is not completely eliminated.

I shall now describe (at) why there is still a small change in g withheater power, and (b) how a further extension of my invention allows meto produce tubes in which the change in g with heater power is zeroor-if I should desire to do soa tube could be made in which an increasein heater power produces a decrease in g It is known that when thecurrent in a tube is limited by the space charge, a virtual cathode isformed at a plane somewhere between the cathode and grid. The virtualcathode is separated from the actual cathode by a distance x and thepotential of the virtual cathode is V volts below that of the actualcathode. As described in my above-mentioned paper, an increase incathode temperature increases'V this is equivalent to both a decrease innegative grid bias and an increase in anode voltage. As a result, thecurrent i and transconductance g increase. Furthermore, an increase inheater power increases x,,,, and this is equivalent to decreasing thecathode-grid spacing s as previously noted, decreasing s increases i andg To nullify the change in g by change in heater power produced by bothdecreasing s and increasing V and x I replace that part of tubularmember 20 situated above the glass by a barrel-shaped member of bi-metal(for example, copper and Invar) construction. Such a member, as shown inFIG. 2, can be formed by welding strips 26 of bi-metal (for examplecopper and Invar) to collars as shown in FIG. 2, or it can be one-piececonstruction. The inside element 30 of the bi-metal (Invar) has a lowercoefiicient of expansion than the outside element 28 (copper). Inoperation, an increase in cathode heating power raises the temperatureof the bi-metal member and the strips bow outward. Consequently, thecathode moves downward and s is increased. By proper choice of materialsand dimensions, the effect of the decrease in S, can be made to balancethe effects of the increase in V and x It will be understood by thoseskilled in the art that high-frequency currents on the strips 26 caninduce small currents to flow in the heater coil 24, and that the efiectof these currents can be neutralized by inserting a quarterwave stopbelow the vacuum seal .25.

What is claimed is:

1. An electron discharge tube having, within an evacuated envelope,separate cathode and anode electrodes having electrode supportingmembers and a control grid interposed between the electrodes, at leastone of said supporting members having a negative coefiicient of thermalexpansion, said supporting member including a hollow tube and aplurality of bimetal strips securing said supporting member to theelectrode supported thereby.

2. An electron discharge tube having, within an evacuated envelope,separate cathode and anode electrodes having electrode supportingmembers and a control grid interposed between the electrodes, at leastone of said supporting members having a negative coeflicient of thermalexpansion, said supporting member including a hollow tube and aplurality of bimetal strips securing said supporting member to theelectrode supported thereby, each strip having an outside element and aninside element, the inside element having a lower coeificient of thermalexpansion than the outside element, and the other supporting membershaving a negligibly small coefficient of thermal expansion.

3. An electric discharge tube comprising an evacuated envelope, ananode, a hollow anode supporting member fastened to said anode andprojecting from one end of said evacuated envelope, said anodesupporting member having a negative coefficient of thermal expansion, acathode, a cathode supporting member projecting from the other end ofsaid evacuate-d envelope, a plurality of bimetal strips coupling saidcathode to said cathode supporting member, each strip having an outerelement and an inner element, the outer element having a differentcoefficient of expansion than the inner element, and a control gridattached to said evacuated element, said control grid being interposedbetween said cathode and anode.

References Cited in the file of this patent UNITED STATES PATENTS2,146,365 Batchelor Feb. 7, 1939 2,380,496 Beard July 31, 1945 2,527,127Gormley Oct. 24, 1950 2,599,395 Kohl June 3, 1952

